Most wetlands systems are considered to be free water surfaced where the surface of the water is exposed to the atmosphere. These include the natural wetlands, such as bogs which are primarily vegetation mosses, swamps that are primarily vegetation and trees, and marshes that are primarily vegetation grasses and emergent macrophytes. (Macrophytes are plants that are rooted in shallow water with their vegetative parts above the surface of the water. They are extremely productive because their roots are in the sediments and their photosynthetic parts are in the air.) These natural systems cannot be used for disposal of effluent from sewage treatment plants. However, if a system is specially constructed for the use of an advanced wastewater treatment plant for tertiary polishing of the effluent, it must be used safely.
Constructed Free Water Surface Wetlands
(See endnote 36)
This type of system mimics the systems that are naturally occurring in marshes. The effluent flows over a vegetated soil surface from a point of inlet to a point of outlet. The liquid may be removed by evapotranspiration or seepage downward through the constructed material. To discharge to a natural source requires a permit which satisfies the National Pollutant Discharge Elimination System limits. Therefore, very few natural systems exist. The size of the units can vary from that which would be used on a single property to treat septic tank effluents to something as large as 40,000 acres. The system usually has one or two shallow basins or channels with barriers so that the effluent will not seep into groundwater and a submerged soil layer to support the roots of the plants. There is a uniform distribution under very low velocity that resembles laminar flow of the effluent which allows the accumulated materials including the live plants, dead plants, and litter to help process the fluid. In the United States, commonly, the effluent gets some sort of treatment before it is released to the constructed free water surface wetlands. The advantages of the system are that: the wetlands system produces no residual biosolids, BOD, chemical oxygen demand (COD), total suspended solids, metals, or persistent organics; the system works effectively with a minimum of equipment and energy use; and it provides green spaces for the community. The disadvantages of the system are: the need for very large areas to process the effluent; cold climate problems; mosquito and other insect vector concerns; increased bird populations which may affect airports that are nearby; and potentially incomplete removal of fecal coliform.